This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
Further, the twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the production of cast strip.
During casting, the casting rolls rotate such that metal from the casting pool solidifies into shells on the casting rolls that are brought close together at the nip to produce a solidified cast strip below the nip. The gap between the casting rolls is such as to maintain separation between the solidified shells at the nip so that semi-solid metal is present in the space between the shells through the nip, and is, at least in part, subsequently solidified between the solidified shells within the cast strip below the nip.
When semi-solid metal between the shells below the nip is mushy, the metal can drip from the edges of the cast strip. This is known as “edge loss.” Even before edge loss occurs, the latent heat of the mushy metal can also cause reheating, and through the effect of the ferrostatic head of the pool, enlargement of the edge portions of the strip. This is referred to as “edging up” and “edge bulge.” To avoid such edging up and edge loss, it was previously proposed to shape the crown of the casting roll to squeeze the shells forming the strip at the edges, and alternatively or in addition, alter the cooling rate, so that solid fraction at the center of the strip within 50 millimeters of the edge of the strip is greater than the fluid critical solid fraction of the metal. See U.S. Pat. No. 6,079,480 and EP 0788854. These approaches involved lowering the temperature of the cast strip within 50 millimeters of the strip edges so the edges of the strip do not contain mushy metal. The '480 patent defines the fluid critical solid fraction as the solid fraction (i.e., the solid phase per unit volume at the center of the strip thickness) does not have fluidity and begins to have strength. This approach also reduced loss of metal from additional edge trimming due to edging up, and thus increasing process efficiency.
The present disclosure provides a completely different approach to improving edge quality during casting by purposely allowing and controlling edging up or edge bulge within 50 millimeters of the strip edges. We have found that the temperature of the strip near the edges can be increased relative to the center portion of the strip width when the metal between the shells near the edges of the cast strip is mushy, i.e., the metal has fluidity and causes edging up of the thin cast strip. We have found that maintaining a higher temperature at the edge portion and controlling edging up improves the edge quality of the cast strip. A method is disclosed for continuously casting metal strip comprising steps of:                assembling a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip therebetween through which thin cast strip may be cast, a metal supply system capable of delivering molten steel above the nip,                    the casting rolls having a crown shape so that edge portions of the cast strip within 50 millimeters of edge of the cast strip have a higher temperature than the cast strip in center portions of the strip width;                        forming a casting pool of molten steel supported on the casting surfaces above the nip in a casting area and controlling side dams adjacent the ends of the nip to confine the casting pool; and        forming a cast strip such that the edge portions of the cast strip within 50 millimeters of each edge of the cast strip is of a higher temperature than the cast strip in the center portions of the strip width.        
The temperature of the strip may be measured at the surfaces of the edge portions and the center portion of the strip by a pyrometer(s). The temperature of the edge portions of the cast strip may be about 10° C. or more higher than the cast strip in the center portions of the strip width. Alternately or in addition, the temperature of edge portions of the cast strip may be about 25° C. or more higher than the cast strip in the center portions of the strip width, or may be about 50° C. or more higher than the cast strip in the center portions of the strip width.
The method may include the step of forming the cast strip such that the center portions of the strip within 50 millimeters (about 2 inches) of each edge of the cast strip have a mushy metal between solidified shells. Alternately, the center portions of the strip within 60 millimeters (about 2.4 inches) of each edge of the cast strip may have a mushy metal between the shells. The edge portions of the cast strip may have a higher temperature within about 60 millimeters of edges of the cast strip than the strip in center portions of the strip width. Further, the method may include the step of controlling the amount of mushy metal between the shells below the nip to control and maintain a limited edging up or edge bulge as desired. Such edging up typically may be rolled out at a hot rolling mill downstream of the casting rolls.
The method of continuously casting metal strip may include the step of controlling a groove formed into at least one side dam by the cast strip by the edge of cast strip during casting to a depth of less than about 2.5 millimeters (about 0.098 inch). Alternately, the method may include controlling the groove to less than about 1.5 millimeters (about 0.059 inch) in depth. Alternately or in addition, the method may include the step of causing the side dam actuator to move the side dam toward the end of the casting rolls when a groove formed in at least one side dam by the cast strip during casting is greater than about 2.5 millimeters. Such side dam wear can be controlled to inhibit edge loss.
An apparatus is disclosed for continuously casting metal strip comprising:                a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a nip therebetween through which thin cast strip may be cast, the casting rolls having a crown shape so that each edge portion of the cast strip within 50 millimeters of edge of the cast strip have a higher temperature than the cast strip in center portions of the strip width; and        a metal delivery system capable of delivering molten steel above the nip and forming a casting pool of molten steel supported on the casting surfaces above the nip in a casting area;        a side dam adjacent each end of the casting rolls at the nip to confine the casting pool; and        a side dam actuator at each end of the casting rolls capable of positioning the side dams during casting and controlling a groove formed into at least one side dam by the cast strip during casting to less than about 2.5 millimeters.        
The crown shape of the casting rolls may be capable of forming a cast strip of steel such that the edge portions of the cast strip within 50 millimeters of each edge of the cast strip is of a higher temperature than the cast strip in the center portions of the strip width. Alternately or in addition, the crown shape of the casting rolls in conjunction with the shell thickness at the nip and casting roll biasing force is capable of forming the cast strip such that edge portions of the strip within 50 millimeters of each edge of the cast strip has mushy metal between the shells to cause edging up.
The crown shape of the casting rolls in combination with the casting roll biasing force may be capable of forming a cast steel strip such that the edge portions of the cast strip may have a higher temperature within about 60 millimeters of edges of the cast strip than the strip in center portions of the strip width. The temperature of the edge portions of the cast strip may be about 10° C. or more higher than the cast strip in the center portions of the strip width. Alternately or in addition, the temperature of edge portions of the cast strip may be about 25° C. or more higher than the cast strip in the center portions of the strip width, or may be about 50° C. or more higher than the cast strip in the center portions of the strip width.
Each side dam actuator may be capable controlling the wear rate of the side dam to control the depth of the groove to less than 2.5 millimeters or 1.5 millimeters to reduce and control edge loss.